A distance measuring apparatus that uses a laser beam launches the laser beam in the form of pulses (hereinafter referred to as a “pulse laser beam”), and detects the laser beam reflected from a target. A distance to the target is measured from a TOF (Time Of Flight) ΔT of the laser beam that is launched from the distance measuring apparatus, reflected by the target, and returned and received by the distance measuring apparatus. When a velocity c of light is regarded to be approximately 300,000 km/s, the distance to the target can be obtained from (c×ΔT)/2, for example.
A measured result of the distance measuring apparatus may in some cases be output in the form of a range image (or depth map) in which distance values at each of range (or distance) measurement points are arranged in an order of the raster scanned samples, for example. The range image may represent the distances to the range measurement points by pixels having colors according to the distances. In the range image representing the distances to each of the range measurement points, it is possible to measure an accurate TOF value at an inner part or the like of the target, because the entire laser beam hits the target and is reflected by the target.
On the other hand, in the range image representing the distances to each of the range measurement points, a contour part of the target corresponds to a boundary part between the target and a background. At the boundary part, only a portion of the laser beam hits the target and is reflected by the target. For this reason, compared to the case in which the entire laser beam hits and is reflected by the target, an amount of the laser beam reflected from the boundary part is smaller, and a rise in an output signal waveform of a photodetector that detects the laser beam is more gradual. In addition, an amplitude of the output signal waveform of the photodetector that detects the laser beam reflected by the boundary part is lower compared to the case in which the entire laser beam hits and is reflected by the target, and a timing when the amplitude of the photodetector output exceeds a threshold value for judging the time when the laser beam reaches the target is delayed. Furthermore, compared to the case in which the entire laser beam hits and is reflected by the target, the measured TOF value becomes larger by an amount corresponding to the delay in the amplitude of the photodetector output exceeding the threshold value. For these reasons, the TOF value that is measured by detecting the laser beam reflected from the boundary part of the target becomes larger than the TOF value representing the actual distance from the boundary part. In this case, when the range image representing the distances to each of the range measurement points is generated based on the measured TOF values, a group of pixels generated from the TOF values corresponding to the boundary part of the target appear as noise. On the other hand, it is impossible to distinguish, simply from the magnitudes of the TOF values, whether the output signal waveform of the photodetector represents the distance to the target or the distance from the boundary part of the target.
In this specification, in the range image representing the distances to each of the range measurement points, the noise caused by the boundary part of the target is referred to as “edge noise”. When a three-dimensional image of the target is generated based on the range image representing the distances to each of the range measurement points, for example, the edge noise appears behind the target in the three-dimensional image.
According to the conventional distance measuring apparatus, it is impossible to distinguish whether the output signal waveform of the photodetector represents the distance to the target or the distance from the boundary part of the target. Consequently, it is difficult to detect the edge noise from the range image representing the distances to each of the range measurement points.
Related art includes Japanese Laid-Open Patent Publications No. 2005-242488 and No. 2014-035302, for example.